Laser locking and self filtering device

ABSTRACT

In accordance with the present invention, an optical device is provided which is used to lock the output of an optical transmitter as well as being employed as a self-filtering element within an optical system. An optical transfer element, coupled along the optical transmission path, either reflects or transmits a portion of the light signal associated with the desired channel wavelength. A portion of the optical signal is supplied to a circuit which controls the output of the transmitter.

FIELD OF INVENTION

The present invention generally relates to optical communication systemsand more particularly to an apparatus for controlling the output of anoptical transmitter.

BACKGROUND OF INVENTION

Wavelength division multiplexing (WDM) is a technique for increasing thecapacity of existing fiber optic networks by transmitting a plurality ofchannels over a single waveguide medium. WDM systems typically include aplurality of transmitters for transmitting modulated information signalson a designated one of a plurality of optical channels or wavelengths.The channels are combined by a multiplexer at a first terminal andtransmitted to a demultiplexer at a receiving terminal where thechannels are separated. One or more amplifiers positioned along atransmission fiber may be used to amplify the transmitted signal. Theseparated channels are then supplied to receiving circuitry whichconverts the optical signals into electrical signals for processing.

The transmitters used in WDM systems typically include semiconductorlasers each transmitting on a designated one of a plurality ofwavelengths typically within the 1550 nm range. The output signal ofeach semiconductor laser is controlled by an associated drive currentsuch that the transmitter output is locked to a particular channelwavelength and modulated with communication information either directlyor externally. Since the transmitted wavelengths are relatively close toeach other, the laser transmitters must be precisely controlled in orderto insure integrity of the communication information. Channel spacingsin a WDM system are typically within the 100 GHz range. With theemployment of more advanced wavelength separation devices, 50 GHz andeven 25 GHz spacings may also be employed. Because these channelspacings are so close together, if the laser transmitter should loselock during operation, the output signal of one channel may interferewith an adjacent channel, thereby corrupting the transmittedcommunication signal.

When a laser transmitter used in a WDM system is first turned on, itexperiences a ramp-up period where the drive current must first increaseto a level where the semiconductor laser provides light at the desiredwavelength and at peak power. During this ramp-up period, the lasertransmitter is not yet operating at the desired wavelength, howeverlight still propagates down the transmission fiber. In addition, duringlaser transmitter operation, the laser output may drift off-channelallowing unwanted light to propagate down the fiber. In either case,this unwanted light transmission may impact adjacent channeltransmission and/or may compromise the integrity of the transmittedinformation signals.

Thus, there is a need to provide an optical device which locks thewavelength output of a laser transmitter in an optical transmissionsystem as well as being configured as a self filtering device forpreventing unwanted light signals to propagate down a transmissionfiber.

SUMMARY OF INVENTION

The present invention meets these needs and avoids the above-referenceddrawbacks by providing an optical device for supplying signals forcontrolling the output of an optical transmitter and providing a selffiltering means for preventing unwanted light signals from propagatingdown a transmission fiber. The optical device in accordance with thepresent invention includes a source of light and an optical transferelement having a first port coupled to the light source. The first portreceives light from the light source. The optical transfer elementincludes a second port for outputting the light and for receiving, as aninput, a first portion of the light associated with a particularwavelength. A filtering element is coupled to the second port of thetransfer element. The filtering element has a transmissivitycharacteristic and a reflectivity characteristic as a function ofwavelength. The filtering element is configured to receive the lightfrom the source of light and reflect the first portion of the lighttoward the second port of the transfer element. The filtering element isalso configured to transmit a second portion of the light that isassociated with one or more wavelengths outside of the particularwavelength. The transfer element also includes a third port foroutputting the first portion of the light reflected back from thefiltering element. A photodetector is coupled to the filtering elementand receives the portion of light transmitted through the filteringelement. The photodetector generates a sense signal in response thereto.A control circuit is coupled to the photodetector and to the source oflight. The control circuit receives the sense signal from thephotodetector and generates a control signal, based on the sense signal,for controlling the light outputted from the source of light.

The foregoing, and other features and advantages of the presentinvention, will be apparent from the following description, theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of a fail safe laser lockingapparatus in accordance with the present invention.

FIG. 2 illustrates a graph of transmittance and wavelength of anexemplary filtering element utilized in an optical device in accordancewith the present invention.

FIG. 3 is a schematic view of an alternative embodiment of an opticaldevice in accordance with the present invention.

FIG. 4 is a schematic view of an alternative embodiment of an opticaldevice in accordance with the present invention.

FIG. 5 is a schematic view of an alternative embodiment of an opticaldevice in accordance with the present invention.

FIG. 6 illustrates a graph of transmittance and wavelength of anexemplary filtering element utilized in an optical device in accordancewith the present invention.

FIG. 7 is a schematic view of an alternative embodiment of an opticaldevice in accordance with the present invention.

DETAILED DESCRIPTION

In accordance with the present invention, an optical apparatus is usedto lock the wavelength output of a laser transmitter as well asproviding a self filtering configuration for preventing the transmissionof an optical signal outside the desired channel wavelength frompropagating down an output fiber. An optical transfer element coupledalong a transmission path of an optical communication system receives anoptical signal having a particular wavelength from a light source ortransmitter. The transfer element includes at least one filteringelement which either reflects or transmits, depending upon theconfiguration, the signal having the desired channel wavelength to anoptical path. If the signal is not within the desired wavelength, thesignal is prevented from propagating down the optical path. A portion ofthe signal is supplied to circuitry which controls the output of thetransmitter.

FIG. 1 schematically illustrates an optical device 10 in accordance withthe present invention wherein transmitter 20, such as a semiconductorlaser, is coupled to the input 26 of optical transfer element 30 viatransmission path 25. Amplifiers can be coupled along path 25 toincrease the signal power and transmission distance of the output oftransmitter 20. Optical transfer element 30 includes circulator 31 andfiltering element 40. Circulator 31 has first, second and third ports32, 33 and 34, respectively and is configured such that optical signalswhich enter port 32 exit through port 33 and optical signals which enterport 33 exit through port 34.

The first circulator port 32 receives the optical signal produced bytransmitter 20 and carried via optical path 25. Circulator 31 andtransmitter 20 can be located within a particular node or module withina communications network. The signal enters circulator 31 at port 32 androtates, in a clockwise direction toward port 33. The signal exitscirculator port 33 and is received by a filtering element 40, tuned toreflect one or more particular wavelengths. The filtering element 40 canbe, for example, a Bragg-grating which is coupled to port 33 by way ofoptical path 35.

The transmittance vs. wavelength spectrum of an exemplary filteringelement 40 is shown in FIG. 2. Filtering element 40 is tuned to have alow transmissivity and high reflection characteristic at wavelengthλ_(o) and a high transmissivity or pass-through characteristic atwavelengths other than λ_(o). Accordingly, a first portion of the lightsignal received from transmitter 20, for example having wavelength λ₀,is reflected by filtering element 40 back to circulator port 33. Thereflected portion of the signal travels clockwise in circulator 31toward circulator port 34 and exits transfer element 30 at output 37onto optical path 36. A second portion of the signal, denoted in FIG. 2as portions 80 and 80', is transmitted through filtering element 40 to amonitoring point at photodetector 45 via optical path 42 from transferelement output 38.

Photodetector 45 receives the second portion of the signal which isoutside the reflectivity bandwidth of filtering element 40 and generateselectrical sense signals in response thereto. These signals are suppliedto control circuitry 50 via line 48. Control circuitry 50 is coupled totransmitter 20 via optical path 49. Circuitry 50 controls the output oftransmitter 20. For example, where transmitter 20 is a semiconductorlaser, circuitry 50 generates output signals which control the drivecurrent supplied to the laser in transmitter 20, thereby adjusting theoutput of the laser to remain within operating bandwidth 81 shown inFIG. 2. Control circuitry 50 can include hardware and/or softwareconfigurations. Examples of these types of control circuitry areincluded in application Ser. No. 08/848,423 entitled "Laser WavelengthControl Device" assigned to the assignee of the present invention and anapplication entitled "Laser Wavelength Control Under Direct Modulation"filed on Jul. 22, 1997 (Unofficial Ser. No. 08/898,714) also assigned tothe assignee of the present invention, both of which are incorporatedherein by reference.

Alternatively, an additional monitoring point can also be includedwithin optical device 10 at photodetector 45'. A portion of the signaloutputted onto line 36 may be tapped via line 36' and supplied tophotodetector 45' where an electrical sense signal is generated inresponse thereto. The sense signal is supplied to control circuitry 50via line 41, and used to control the output of transmitter 20 asdescribed above.

If a portion of the output wavelength of transmitter 20 is not withinwavelength λ_(o), i.e. the signal drifts off-channel, that portion ofthe signal is transmitted through filtering element 40 rather than beingreflected back through port 33. Likewise, when transmitter 20 is firstturned on, it experiences a ramp-up period where a portion of the outputwavelength has not reached the desired wavelength, e.g. λ_(o). Thisunwanted signal portion is prevented from propagating down optical path36 by way of filtering element 40 reflecting wavelength λ_(o) backtowards port 33 of circulator 31 and out port 34. In this manner,optical device 10 provides a transmitter locking device as well as aself-filtering configuration which prevents off-channel wavelengths frompropagating down transmission path 36.

FIG. 3 is a schematic illustration of an alternative embodiment ofoptical device 10' in accordance with the present invention wheretransmitter 20 is coupled to the input 26 of optical transfer element 30via a transmission path 25. Optical transfer element 30 includes anoptical coupler 43, for example a fused fiber coupler, having an inputpath 44 for receiving the light signal from transmitter 20. Coupler 43taps a percentage of the input signal from transmitter 20 and directsthe signal onto input/output path 46. Filtering element 40 is coupled toinput/output path 46 of coupler 43 and receives the signal tapped ontopath 46 of coupler 43. Filtering element 40 can be, for example, aBragg-grating configured to have a low transmissivity and highreflection characteristic at wavelength λ_(o), and a high transmissivityor pass-through characteristic at wavelengths other than λ_(o) similarto the filter spectrum shown in FIG. 2. A first portion of the lightsignal having wavelength λ_(o) is reflected by filtering element 40 anddirected toward path 47 of coupler 43 via input/output path 46. In thismanner, the signal having wavelength λ_(o) exits transfer element 30 atoutput 37 via coupler path 47.

The remaining portion of the signal received on input/output path 46which is outside of wavelength λ_(o) is transmitted through filteringelement 40 to a monitoring point at photodetector 45 via line 42.Photodetector 45 receives this second portion of the signal which isoutside the reflectivity bandwidth of filtering element 40 and generatesan electrical sense signal in response thereto. This sense signal issupplied to control circuitry 50 via line 48. Control circuitry 50 iscoupled to transmitter 20 via optical path 49 and controls the output oftransmitter 20 as described above. Alternatively, an additionalmonitoring point can also be included within optical device10' atphotodetector 45'. A portion of the signal outputted onto line 36 may betapped via line 36' and supplied to photodetector 45' where anelectrical sense signal is generated in response thereto. The sensesignal is supplied to control circuitry 50 via line 41 and used tocontrol the output of transmitter 20 as described above.

Referring to FIG. 4 which is a schematic illustration of an alternativeembodiment of optical device 100 in accordance with the presentinvention, where transmitter 20 is coupled to the input 26 of opticaltransfer element 30 via a first transmission path 25. Optical transferelement 30 includes a first and second optical couplers 60 and 65 withnarrow band filtering elements 70 and 75 disposed therebetween. Each ofthe couplers 60 and 65 can be, for example, fused fiber couplers. Thelight signal from transmitter 20 is received by coupler 60 on line 61.For purposes of illustration, couplers 60 and 65 tap 50% of the inputsignal from transmitter 20 in essence splitting the received signal,however tapping percentages can vary. A first portion of the signalreceived on line 61 is transferred, with a 90° phase-shift, to line 62and is carried to filtering element 70. The other portion of thereceived signal from transmitter 20 continues on line 61 to filteringelement 75.

Filtering elements 70 and 75 can be, for example, Bragg-gratingsconfigured to have a low transmissivity and high reflectioncharacteristic at wavelength λ_(o), and a high transmissivity orpass-through characteristic at wavelengths other than λ_(o) similar tothe filter spectrum shown in FIG. 2. A first portion of the light signalhaving wavelength λ_(o) is reflected by filtering element 75 andtransferred to line 62 with a 90° phase-shift. Likewise, a secondportion of the signal having wavelength λ_(o) is reflected by filteringelement 70 and carried on output line 62. Thus, the signal havingwavelength λ_(o) exits transfer element 30 at output 37 via line 62.

The remaining portions of the signal received from transmitter 20outside of wavelength λ_(o), are transmitted through filtering element70 and 75 to a monitoring point at photodetector 45 via line 42. Forexample, a third portion of the signal, not reflected by filteringelement 70 is first phase-shifted 90° when it jumps from line 61 to 62.This same signal receives another 90° phase shift when it jumps fromline 76 to 77. Thus, the total phase shift for this third portion of thesignal is 180° as compared with a fourth portion of the signaltransmitted through filtering element 75 on line 61 which has a 0° phaseshift thereby canceling-out both the third and fourth signal portions.However, a fifth portion of the signal which jumps from line 61 to 62,transmitted through filtering element 70 and carried on line 76 onlyincurs a 90° phase shift and is received by photodetector 45. Likewise,a sixth portion of the signal carried on line 61 and transmitted throughfiltering element 75, jumps to line 76, thereby incurring a 90° phaseshift. This fifth portion of the signal as well as the sixth portion ofthe received signal from transmitter 20 are in phase and received atphotodetector 45.

Photodetector 45 receives these signals which are outside thereflectivity bandwidth of filtering elements 70 and 75 and generates anelectrical signal in response thereto. This sense signal is supplied tocontrol circuit 50 via line 48. Control circuit 50 is coupled totransmitter 20 via optical path 49. As described above, circuit 50controls transmitter 20 locking the transmitter output at wavelengthλ_(o). Alternatively, an additional monitoring point can also beincluded within optical device 100 at photodetector 45'. A portion ofthe signal outputted onto line 36 may be tapped via line 36' andsupplied to photodetector 45' where an electrical sense signal isgenerated in response thereto. The sense signal is supplied to controlcircuitry 50 via line 41, and used to control the output of transmitter20 as described above.

If a portion of the output wavelength of transmitter 20 is not withinwavelength λ_(o), i.e. the signal drifts off-channel, that portion ofthe signal is transmitted through filtering elements 70 and 75 ratherthan being reflected back through transfer element output 37 via line62. Likewise, when transmitter 20 outputs a signal whose wavelength isnot the selected channel, for example λ_(o), the signal is preventedfrom propagating down optical path 36 by way of filtering elements 70and 75 only reflecting wavelength λ_(o) back towards port transferelement output 37. In this manner, optical device 100 provides atransmitter locking device as well as a self-filtering means whichprevents offchannel wavelengths from propagating down transmission path36.

Referring to FIG. 5 which is a schematic illustration of an alternativeembodiment of optical device 200 in accordance with the presentinvention, where transmitter 20 is coupled to optical transfer element230 via a first transmission path 25. Optical transfer element 230includes a splitter 210 coupled to a filtering element 215. The lightsignal from transmitter 20 passes first optical splitter 210 and isreceived by filtering element 215 via line 211.

Filtering element 215 is configured to have a high transmissivitycharacteristic at particular wavelengths, λ_(o). . . λ_(N) and lowtransmissivity and reflection characteristic at wavelengths other thanλ_(o). . . λ_(N) as described in more detail below. Filtering element215 can be, for example a high finesse Fabry-Perot filter. The lightsignal received from splitter 210 via line 211 having a particularwavelength, for example λ_(o), is transmitted through filtering element215 to line 216 and propagates down optical path 36. Filtering element215 reflects the remaining portion of the light signal received fromtransmitter 20 which is outside wavelength λ_(o) and supplies it tosplitter 210 where it is directed to output line 212. Photodetector 240receives this portion of the signal from splitter 210 and generates anelectrical sense signal in response thereto. Control circuit 50 iscoupled to photodetector 240 by way of line 48 and to transmitter 20 byway of line 49. Control circuit 50 receives the sense signal andgenerates a control signal, based on the received sense signal, forcontrolling the signal output of transmitter 20. As described above,circuit 50 is used to lock the output of transmitter 20 to a particularwavelength.

Filtering element 215 is configured to have a high transmissivity atparticular wavelengths and low transmissivity at other wavelengths. FIG.6 illustrates a transmission spectrum 300 for odd numbered channels in aWDM systems using a Fabry-Perot filter of the type referred to withreference to FIG. 5. This type of filter, manufactured by Dicon Corp.,has 100 GHz free spectral range with 3 dB minimum loss and 10-20 dBrestriction. The Fabry-Perot filter employed as filtering element 215has 200 GHz free spectral range for a 100 GHz channel spaced WDM system.Thus, a WDM system using this type of Fabry-Perot filter can employ onefiltering element for the odd numbered channels λ₁, λ₃, etc. and one forthe even numbered channels λ₂, λ₄, etc. In this manner, the risk ofhaving transmitter 20 lock to the wrong channel is reduced because theadjacent channel is at a minimum as indicated by spectrum portion 350.For example, transmitter 20, such as a semiconductor laser, using theoptical device in accordance with the present invention locks onto anoptical channel having wavelength λ₁. By using a filtering element forthe odd channels and another for the even channels, the risk oftransmitter 20 locking onto a channel having wavelength λ₃ issignificantly reduced because the adjacent channel having wavelength λ₂is at a minimum (spectrum portion 350) and the control circuitry used tolock transmitter 20 is configured to detect such a significant channeldeviation. Thus, if transmitter 20 drifts off-channel or outputs asignal not within the selected wavelength for that particular channel,optical device 200 acts as a laser locking apparatus as well as a selffiltering means and prevents the unwanted signals from propagating downoptical path 36.

Referring to FIG. 7 which is a schematic illustration of an alternativeembodiment of optical device 200' in accordance with the presentinvention, where transmitter 20 is coupled to optical transfer element230 via a first transmission path 25. Optical transfer element 230includes a first optical splitter 210, a second optical splitter 220 anda filtering element 215 disposed between first splitter 210 and secondsplitter 220. The light signal from transmitter 20 passes first opticalsplitter 210 and is received by filtering element 215 via line 211.

As described with reference to FIG. 5, filtering element 215 can be, forexample a high finesse Fabry-Perot filter configured to have a hightransmissivity characteristic at particular wavelengths, λ_(o). . .λ_(N) and low transmissivity or reflection characteristic at wavelengthsother than λ_(o). . . λ_(N). The light signal received from splitter 210via line 211 having a particular wavelength (e.g. λ_(o))is transmittedthrough filtering element 215 to splitter 220 via line 216. Splitter 220outputs a portion of the light signal having the particular wavelengthreceived from filtering element 215 for propagation down optical path36. Splitter 220 also outputs a relatively small portion of the lightsignal having the particular wavelength to line 217 which is coupled tophotodetector 245 and which generates an electrical signal in responsethereto. Filtering element 215 reflects the remaining portion of thelight signal received from transmitter 20 which is outside theparticular wavelength (e.g. λ_(o)) and supplies it to splitter 210 whereit is directed to output line 212. Photodetector 240 receives thisportion of the signal from splitter 210 and generates an electricalsignal in response thereto.

Interface circuitry 250 receives the electrical signals fromphotodetectors 240 and 245 from lines 246 and 247, respectively andsupplies them to control circuitry 50 via line 48. Alternatively,control circuit 50 can be configured to receive the electrical signalsdirectly from photodetectors 240 and 245, thereby eliminating interfacecircuitry 250. As described above, circuit 50 is used to lock the outputof transmitter 20 to a particular wavelength.

While the foregoing invention has been described in terms of theembodiments discussed above, numerous variations are possible.Accordingly, modifications and changes such as those suggested above,but not limited thereto, are considered to be within the scope of thefollowing claims.

What is claimed is:
 1. An optical device comprising:a source of light; an optical transfer element having a first port coupled to said source of light, said first port receiving said light, said transfer element having a second port for outputting said light and receiving, as an input, a first portion of said light associated with a particular wavelength, said transfer element having a third port for outputting said first portion of said light so that said first portion of light propagates away from said source of light; a filtering element coupled to said second port of said transfer element, said filtering element having a transmissivity characteristic and a reflectivity characteristic as a function of wavelength, said filtering element configured to receive said light and reflect said first portion of said light toward said second port of said transfer element, said filtering element configured to transmit a second portion of said light associated with one or more wavelengths outside of said particular wavelength; a photodetector coupled to said filtering element, said photodetector receiving said second portion of said light and generating a sense signal in response thereto; and a control circuit coupled to said photodetector and to said source of light, said control circuit receiving said sense signal from said photodetector and generating a control signal, based on said sense signal, for controlling said light outputted from said source.
 2. The optical device in accordance with claim 1 wherein said optical transfer element comprises an optical circulator.
 3. The optical device in accordance with claim 1 wherein said filtering element comprises a Bragg-grating.
 4. The optical device in accordance with claim 1 wherein said optical transfer element comprises an optical coupler.
 5. The optical device in accordance with claim 2 wherein said circulator is configured, within a particular module, with said source of light.
 6. The optical device in accordance with claim 1 wherein said photodetector is a first photodetector and said sense signal is a first sense signal, said optical device further comprising a second photodetector coupled to said third port of said transfer element, said second photodetector receiving a percentage of said first portion of said light and generating a second sense signal in response thereto.
 7. The optical device in accordance with claim 6 wherein said control circuit is configured to receive said second sense signal and generate a control signal, based on said second sense signal, for controlling said light outputted from said source.
 8. The optical device in accordance with claim 6 wherein said control circuit is configured to receive said second sense signal and generate a control signal, based on said first and second sense signals, for controlling said light outputted from said source. 